Preparation of dissolving pulps from wood by hydrolysis and alkaline sulfite digestion



Oct. 6, 1970 K. J. LJUNGQVIST 5 5 OF DISSOLVING PULPS FROM WOOD BY AND ALKALINE SULFITE DIGESTION 3 Sheets-Sheet 2 PREPARATION HYDROLYS IS Filed March 11, 1969 333 mmx\mE2m 00m mt om. mm 00. mn om mm V E m-z 902 5 3 z 6.0: E & O E Pozs om o rtmoum;

v T Q m on ml 00 mm Om m9 ON mm Om m2 003 mmx\mE2m pom mt om m- 00. mm on mm INVFNTOR. KARL JOHhN LJUNGQVIST ATTORNEYS United States Patent U.S. Cl. 162--84 4 Claims ABSTRACT OF THE DISCLOSURE High purity chemical pulps of the dissolving pulp type are produced with enhanced characteristics of high viscosities and alkali-resistant cellulose contents and low pentosan contents by multi-stage cooking or digesting techniques including a substantially non-delignifying prehydrolysis treatment followed by an alkaline cooking or digesting treatment utilizing an alkaline sodium sulfite cooking liquor to which has been added a substantial proportion of sodium sulfide (primarily to achieve the enhanced characteristics of these dissolving pulps), optionally some sodium carbonate, and sodium hydroxide (primarily to increase the viscosity at a low kappa number but also in order to shorten the cooking or digesting time required to achieve the desired high chemical pulp analyses).

This application is a continuation-in-part of copending application Ser. No. 696,006, filed Jan. 5, 1968 being a continuation-in-part of application Ser. No. 325,166, filed Nov. 20, 1963 both now abandoned.

This invention relates to the preparation of chemical pulps of the dissolving pulp type from wood and, more particularly, to cooking and digesting and treating techniques for producing chemical or dissolving pulps of high cellulosic purity, high viscosities and alkali-resistant cellulose contents, and low pentosan contents, with multistage alkaline sulfite cooking or digesting and prehydrolysis techniques.

As will be understood, in the manufacture of cellulosic pulps especially those to be used for certain chemical purposes such as viscose production, as contrasted to wood pulps for paper manufacture, etc.an extremely high degree of cellulosic purity (i.e. minimal pentosan and alkali-soluble cellulose contents) may be desired, along with relatively high viscosities or, even extremely high viscosities for dissolving pulps for nitration, the production of certain types of polynosic fibres and/or high viscosity carboxymethyl celluloses, etc., and to a greater and more critical degree than is required in the production of wood pulp for the paper making industry. Of course, the goals of obtaining high cellulosic purity and low pentosan content are conventionally inconsistent with the goals of obtaining a high viscosity pulp with maximum strength, particularly in a pulp which is to be bleached.

Yet the optimizing of these characteristics may well be a significant factor in producing dissolving pulps for use as starting materials in the chemical cellulose industry as distinguished from the paper making industry. Conventionally, such chemical or dissolving pulps are produced from wood by sulfate cooking or digesting techniques, usually preceded by a prehydrolysis step in which the wood chips are heated under pressure with water or dilute mineral acids in a manner which may remove part of the hemicellulose materials but which is substantially ineffectual for lignin removal, prior to a conventional sulfate 3,532,597 Patented Oct. 6, 1970 cook for dissolving lignin and the major portion of the remaining hemicellulose materials as in the manufacture of wood pulp for paper making.

Alternatively, maximum removal of substantially all the lignin and hemicellulose materials which are to be removed may be attempted using a conventional sulfite cook, as is used with paper making pulps. In either event, various attempts have been proposed to purify the cooked pulp subsequent to the primary digestion or cooking step by heating with alkaline solutions usually diluted sodium hydroxide and/ or sodium carbonate) as well as attempts to obtain the desired high purity with conventional sulfite techniques arrested at the point where most of the lignin in the wood has been sufficiently sulfonated to be capable of being dissolved by alkali, and then injecting alkali into the digestor with or without first removing the sulfite cooking liquor.

Characteristically, as will be understood, in such pulping methods based on one or another variant of the conventional sulfite paper pulp process, the actual cooking or digesting step is initiated with an acid sulfite cooking liquor in a quantity or concentration necessary for the sulfonation and dissolving of the lignin, while the sulfite cook also contains sulfur dioxide and a metal sulfite. Nevertheless, utilizing such techniques, or even combinations thereof may not succeed in producing cellulosic pulps of desired high purities and viscosities and alkali-resistant contents for certain chemical applications as dissolving pulps, and at least not directly as an initial product of the principal cooking and digesting steps to which the wood or other cellulosic materials may be subjected.

Merely for purposes of comparison with this invention, a conventional prehydrolysis sulfate treatment may be characterized as including prehydrolysis of the wood in water for perhaps 50 minutes at 170 C. followed by a sulfate cooking for digesting step for perhaps minutes at 170 C. and with a cooking liquor comprising sodium sulfide and sodium hydroxide in a ratio of, respectively, 70 gms. and gm per kilogram bone dry wood, for the production of a conventional chemical or dissolving pulp. Similarly, in the paper pulp field, but not a dissolving pulp, one may note, for purposes of comparison, in the disclosure of Pat. 1,880,048, a 4-hour prehydrolysis at C. followed by a neutral sulfite cook at about 180 C. for about 6 hours with the cooking liquor including a subordinated proportion of sodium hydroxide, sodium sulfide, or sodium carbonate, etc.

Similarly, the Pat. 1,822,126 discloses a method of preparing sulfite pulp for filaments and films according to which the pulp prepared by an ordinary acid sulfite process dissolving at least 40% of the Wood, washed and usually screen is treated with a solution containing sodium sulfite, sodium hydroxide and sodium sulfide for a period between 2 and 3 hours at a temperature of about 140 C., the amounts of the active alkali used (sodium sulfide plus sodium hydroxide) being more than 300 gms. on every kg. air dry pulp.

According to this invention, by contrast, chemical pulps of the dissolving pulp type are produced with a chemical purity considerably exceeding that obtained by the noted conventional techniques (either for dissolving pulps or for paper making pulps and whether or not they include a prehydrolysis step and whether they relate to the sulfite or sulfate cooking methods), and such enhanced results are achieved in accordance herewith by the utilization of a conventional prehydrolysis pretreatment step at about ISO-180 C. which is not a delignifying or sulfonating prehydrolysis, followed by a subsequent cooking or digesting at about -180 C. utilizing an alkaline solution of sulfite with a sulfide component and with extra free hydroxide present to increase the viscosity and to reduce the cooking time. At least 50% of the active chemicals in the cooking liquor is preferably sulfite, while sulfide should be present at least in the amount of 40 gms. per kilo of bone dry wood, and at least 20 gms. hydroxide per kilo of bone dry wood, with the sum of sulfide and hydroxide being within the range of about 60-180 gms. per kilo of bone dry wood. The spent cooking liquor chemicals from this process are readily recovered for re-use, and the cooking liquor may optionally contain a substantial proportion of carbonate above and beyond that which is normally present in a conventional sulfite or sulfate pulping procedure.

With the foregoing and additional objects in view, this invention will now be described in more detail, and other objects and advantages will be apparent from the following description, the accompanying drawings, and the appended claims.

In the drawings:

FIG. 1 is a graphic representation of data comparing the cooking or digesting time in accordance herewith to attain a kappa number of 36 plotted against the total active alkali (in gms./kgs. bone dry wood) at 180 C. digesting temperature, in which (as with all the other drawings herein) the open circles indicate a sulfite cook with sodium hydroxide added thereto, the black circles indicate a sulfite cook with sodium sulfide added thereto, and the half-filled circles indicate a sulfite cook with the preferred combination of both hydroxide and sulfide added thereto, and the X indicates a plot of data from a run made in accordance with the aforementioned paper pulp patent process and the squares indicate a plot of data from a run in accordance with the aforementioned conventional prehydrolysis sulfate process;

FIG. 2 is a similar graphic representation of plotted data comparing the pentosan content of various runs with varying amounts of active alkali and at various prehydrolysis conditions, with all pulps cooked to a kappa number of 36;

FIG. 3 is a similar graphic representation plotting the maximum level obtainable in alkali-resistance cellulose using a prehydrolysis in accordance herewith of 4 hours at 150 C. for alkali solubilities of 10% and 18% NaOH (noted as R10 and R18 respectively, determined as noted herein);

FIG. 4 is a similar graphic plot of data comparing the viscosities at varying amounts of active alkali for various runs utilizing different prehydrolysis conditions and showing in a spectacular way the big difference between runs with mixture of sulfide and hydroxide in contrast to either of them alone; and

FIG. 5 is a graphic representation of plotted data indicating a comparison of viscosity and kappa number for the various runs noted, it being understood that, in addition to the legends appearing on the various drawings, the aforementioned coding of the data plotted is uniform throughout the various figures thereof.

Generally in accordance herewith, the prehydrolysis step is controlled, as by the formulation of the ingredients as well as operation controls of time and temperature, to be of a nature which avoids substantial delignification or sulfonation of the wood being treated, while the sulfite cooking step utilizes a cooking or digesting liquor which also includes a significant proportion of sulfide, and hydroxide material. The prehydrolysis step in accordance herewith is carried out at a temperature of about 150- 180 C. utilizing only water (which may be in the form of steam, as an alternative to actually soaking the wood chips in hot water). The hydrolyzing intensity is such that -25%, normally 12-15%, of the wood is dissolved in case of soft wood and normally 15-25% in case of hard wood. The hydrolyzing intensity is a function of the temperature and the period of time used. At the given conditions the prehydrolysis requires normally 4-50 minutes, but may be as long as 120 minutes.

As pointed out below, the spent liquor from the alkaline cooking step is satisfactorily used for preparing new cooking liquor, and/or sodium and sulfur materials are satisfactorily recovered from the spent liquor with which to prepare new cooking liquor. In accordance herewith, however, at least of the active chemicals should be soluble sulfite and, regardless of the optional presence of sodium carbonate (which is normally present in the chemicals used to formulate the cooking liquor and may be present in rather substantial proportions without disadvantageously affecting the results obtained), at least about 40 gms./kg. bone dry wood of sodium sulfide must be present with at least about 20 gms./kg. Wood of sodium hydroxide to reduce the cooking time to optionally practical short duration (generally 40-180 minutes) for commercial operation, with the sum of the quantities of sodium hydroxide and sodium sulfide being within the range of about -180 gms./kg. bone dry wood. The lower limit of sodium sulfite of about gms./kg. bone dry wood is critical. Below this limit the wood will not provide a defibrable pulp. However, this lower limit will naturally vary somewhat with the other cooking conditions and with the sort of wood used. A practical lower limit is about 200 gms./kg. wood. Since there is no substantial increase in quality if the sulfite component is above 400 gms./kg. wood, that figure represents, from an economical point of view, a practical, although not critical, upper limit. The quantity of active alkali (sodium hydroxide and sodium sulfide) has a lower critical limit of about 60 gms./kg. bone dry wood. Below this limit it is not possible to obtain the high viscosity characteristics for this new pulp of high quality dissolving pulp type. The upper limit cannot be stated exactly as the pulp qualities in most respects decreases from an optimum at about 130-150 gms./kg. bone dry wood. For the same reasons the upper limit normally is about gms./kg. bone dry wood. It is also necessary that both sodium sulfide and sodium hydroxide are present. It has been shown that the necessary low pentosan content is not obtained with less than about 40 gms./ kg. bone dry wood of sodium sulfide and less than about 20 gms./kg. wood of sodium hydroxide.

The digestion is generally carried out at a temperature of 165-180 C. a preferable range being-170475" C. However, the temperature ranges adapted for the prehydrolysis and the digestion are not critical in the sense that the objects of the invention cannot be obtained outside these limits. For practical reasons, such as e.g. the period of time necessary for each step, the equipment available, these limits are satisfactory. A man of the art can easily determine the period of time necessary for each step. They will be generally within the range of 4-100 minutes for the prehydrolysis and 40-180 minutes for the digestion. They will vary within these limits with the cooking conditions, such as e.g. the temperature, the quantities of active chemicals, the sort of wood, etc.

As will be apparent from the foregoing, and from the data graphically set forth in the drawings hereof, the amount of active alkali (designated as AA in the drawings) in cooking techniques embodying and for practis- 1ng this invention is roughly only about half the quantity which is used in the usual prehydrolysis sulfate process. For this reason, among others, it is quite surprising and unexpected, in term of conventional knowledge and practices in the various paper making pulp cooking techniques or dissolving pulp cooking techniques, with or without prehydrolysis steps, that the alkali solubilities and pentosan contents of dissolving pulps produced in accordance herewith are lower than those obtained by the aforementioned conventional techniques, whether sulfate or sulfite and whether with or without prehydrolysis.

Preferably, the prehydrolysis step in accordance with this invention is carried out by subjecting the wood chips to a treatment with water at about C. for about 20 to 60 minutes, with the wash water then being drained from the wood chips being treated. As will be understood, however, a satisfactory prehydrolysis step in accordance herewith may be accomplished by treating the wood chips with water (including in the form of steam) for a longer time at a lower temperature (e.g. perhaps 4 hours at 150 C. or for a shorter time at a higher temperature (perhaps minutes at 175 C.), depending upon a variety of operational factors. That is, in the various specific examples to which reference is made below, in most cases the wood involved is pine (pinus silveszris), and prehydrolysis of such wood materials to produce satisfactory results in accordance herewith may embody, as will be well understood, differing conditions from the prehydrolysis of other cellulose-base materials to which this invention is satisfactorily applicable, and may even lead to additional variations of the applicable processing or operational conditions depending upon the choice of the operator, etc., yet all without inventive experimentation to achieve results in accordance herewith which produce a satisfactorily enhanced dissolving pulp product. Similarly, whether the prehydrolyzed wood chips are merely separated from the phehydrolysis liquor by draining or washed or whether they are subjected to the prehydrolysis treatment by steaming instead of immersing in heated water appears not to be of significant importance in connection herewith, with such varying operational techniques well within the disclosure hereof and more or less equivalent in achieving the enhanced results of this invention.

As illustrative of the surprising results in accordance herewith, one may note the process for producing a paper pulp (not a dissolving pulp as is hereby produced) from wood by cooking in a weak alkaline liquor containing both sodium sulfite and sodium sulfide as described, for example, in a paper by Peckham and Van Drunen; TAPPI vol. 44 No. 5, pp. 374-384, (1961). A representative composition of the cooking liquor according to that technique contains both sodium sulfite corresponding to about 6% sulfur dioxide, 1% sodium carbonate, and 1% sodium sulfide. Wood pulps digested or produced with such a cooking liquor and according to the teaching of the noted publication provide papers which exceed other papers produced from pulps made by other conventionad digesting methods in bursting strength, tearing strength, and tensile strength, but they are still paper making pulps, rather than dissolving pulps as stressed in accordance herewith. As to chemical purity, such pulps analyze to show a pentosan content of approximately 9% by weight and an alpha cellulose content of approximately 84% i.e., substantially outside the class of dissolving pulps produced in accordance with this invention.

By contrast, applying the techniques in accordance herewith for the manufacture of chemical or dissolving pulps from the pine wood noted above (and/or altering the operating conditions and compositions as may be required for treating other woods in a manner well understood by men skilled in this art) have achieved pentosan contents which are at least 1% to 2% lower than those obtained by conventional prehydrolysis and subsequent sulfate cooking, and the pulps produced in accordance herewith have surprisingly higher viscosities. The same is true comparing pulps in accordance herewith, as noted below, with the sulfite cook of the above mentioned patent on paper making pulps, even when substantial quantities of sulfide, hydroxide, etc., have been added to the sulfite paper making pulp digesting liquor. Actually, pentosan contents below even 0.9% have been obtained in accordance with this invention and thus, as will be understood, at levels of purity which are not conventionally possible with regular prehydrolysis sulfate cooking and/or sulfided sulfite cooking in accordance with the above noted patent, at least without resorting to a further alkaline refining or other purification step in addition to the basic cooking or digesting technique.

Further to compare the techniques embodying and for practising this invention with a conventional prehydrolysis sulfate digesting situation, the operating conditions and pulp analyses set forth in the following Table I may be noted.

TAB LE I Prehydrolysissnlfate Prehydrolyzod at 50 min./170 C Digested at min/170 C NaZSO; (gms/kg. Wood), 0. N328 (gmsJkg. Wood), 70 NaOH (gms./kg. wood), Kappa No., 20i4 Viscosity, 37:1;5... Pentosan, 2.0

0, 95. 4 Yield (unbleached) The present invention 20 min./170 60 min./l70.

50-150 gain/170 Similarly, by way of comparing and contrasting the techniques and results of the instant invention with such conventional prehydrolysis sulfate techniques as well as with the sulfite technique noted in the aforementioned patent and the situations and results obtained when the sulfite cooking liquor in connection herewith is reinforced with only additional sulfide or hydroxide, instead of both together, reference may be made to the drawings hereof.

For example, referring to FIG. 1 and the data plotted thereon, it is readily apparent that the combined addition of sulfide and hydroxide to a sulfite cook in accordance herewith tremendously reduces the digestion time to achieve a given kappa number. From the data in FIG. 1, for example, various times are plotted to achieve a kappa number of 36 at 180 C.,plotted against the active alkali in terms of gms./kgs. bone dry wood. As indicated, with more than gms./kgs. active alkali, the kappa number of 36 is attained even before the maximum digesting temperature of 180 C. is reached. It is also significant to note the almost offscale reading (indicated by the mark X and a square, respectively) for the sulfite process of the patent noted above and the conventional prehydrolysis sulfate process techniques when compared in the same manner with those embodying and for practising this invention. These FIG. 1 curves also indicate the tremendous advantage of using an addition of both sodium sulfide and sodium hydroxide (the half-filled circles), as compared with utilizing only one of these reinforcing materials alone, as well as the important factor in connection herewith whereby the extra addition of sodium hydroxide tremendously shortens the digesting time. That is, the curve for sodium hydroxide addition alone indicates less than optimum results primarily because the increased pH induced into the sulfite cook by addition of sodium hydroxide alone does not reduce the digesting time (without the synergistic addition of sodium sulfide) because the delignifying action of the sodium sulfite cook diminishes at high levels of alkalinity in the absence of the sodium sulfide addition.

As a further comparison or endorsement of the enhanced results achieved in accordance herewith, there are plotted on FIG. 2 similar data relating to the reduced pentosan content of the pulps produced in accordance herewith under varying conditions of prehydrolysis and amounts of active alkali, but with all the pulps tested digested to a kappa number of 36.

The several curves of FIG. 2 relate to a prehydrolysis treatment for 4 hours at 150 C., 50 minutes at C., and 60 minutes at 170 C. in water, with the amounts of active alkali being plotted against the final pentosan con tent when digested at C. to a kappa number of 36. Particular attention may be called to the exaggeratedly high pentosan contents achieved with the normal prehydrolysis sulfate.

As previously noted and as will be well understood, in addition to a low pentosan content and preferably short cooking times, optimum characteristics and results for dlssolving pulps manufactured in accordance herewith include a relatively high viscosity of the unbleached pulp and a relatively high content of alkali-resistant cellulose (or, stated conversely, a relatively low alkali-soluble cellulose content). As indicative of the achieving or such conditions with processes embodying and for practising this invention, there may be noted the data plotted in FIGS. 3 and 4. In the former, the weight percent of alkaliresistant cellulose in the unbleached pulp (digested to a kappa number of 36) is plotted against the active alkali in gms./kg. bone dry wood for a process in accordance herewith in which the prehydrolysis was conducted at 150 C. for 4 hours, with the two curves indicating the respective alkali resistances at 10% and 18% sodium hydroxide R10 and R18, respectively). Regarding FIG. 4, the viscosities of various pulp runs in accordance herewith are plotted against the grams of active alkali per kilo of Wood for various runs having not only varying additions of sodium sulfide or sodium hydroxide, but also various prehydrolysis treatments, although all were digested at 180 C. to a kappa number of 36, including such variations in prehydrolysis all the way from 4 hours at 150 C. to 60 minutes at 170 C., and with the results still compared with those attributable to a comparable run of pulp with the prehydrolysis sulfate conventional technique.

Regardless of what may be the situation with paper making pulps, a low viscosity with a dissolving pulp (especially in the unbleached stage since the viscosity will be additionally lowered during bleaching) is highly undesirable and indicates a severe degradation of the cellulosic content. Generally speaking really preferred dissolving pulps for applications to which this invention is particularly related should have a viscosity (prior to bleaching) of at least over 35 when cooked or digested to a kappa number below 30. The various viscosities as indicated in FIG. 4 for the patented sulfite paper making pulp process noted above (as indicated by the mark X) or this patented process reinforced by a subordinate proportion of sodium hydroxide and sodium sulfide (open circles and filled circles in the lower left corner) make readily apparent that these are much too low for satisfactory results here.

As noted above, regarding viscosities of pulps in the unbleached state produced in accordance herewith, adequately high viscosities are obtained for normal viscose production, and it is even possible to obtain extremely high viscosities as may be required or desired in dissolv- :ing pulps for specialized applications, such as, for example, nitration, polynosic fibre production, high viscosity carboxymethyl cellulose production, etc. As more demonstrative of the foregoing, reference is made to FIG. in which data are plotted indicating the relationship between viscosity and kappa number, as well as the noted pentosan contents, for a variety of pulps in accordance herewith as well as the aforementioned conventional prehydrolysis sulfate, all subjected to a prehydrolysis treatment for 4 hours at 150 C., and with all of the sulfite pulps in accordance herewith including a digesting step utilizing a sulfite cooking liquor charge of about 400 gms./kgs. wood with the additives as noted. As will be apparent from the viscosity/kappa number data plotted in FIG. 5, the sulfide-reinforced sulfite cooking liquor in accordance herewith produces substantially enhanced results with the addition of a significant amount of hydroxide to diminish the cooking or digesting time and, hence the cellulosic degradation which. would occur from harsher or longer cooking cycles even if the shorter were not of important economic significance in commercial scala operations.

For purposes of completeness and illustration, it may appropriately here be noted that the foregoing data and, in the data reported with regard to the specific examples set forth below, the lignin contents were determined and are expressed as kappa numbers according to SCAN C1:59; the viscosities were determined according to TAPPI procedure 206 and calculated from CCA 28:57.

The pentosan contents were determined according to CCA 24:57; the alkali resistances at 10% and 18% sodium hydroxide (noted as R10 and R18, respectively) were determined according to SCAN-C2261; and the temperatures are all expressed in degrees centigrade.

Accordingly, merely as illustrative of techniques embodying and for practising this invention, reference will now be made to the following specific examples, without any limitation thereof on the scope of this invention, and with Example I illustrating a conventional combination of prehydrolysis sulfate cooking techniques for the purposes of comparison with the other examples produced and conducted in accordance with this invention.

Several batches of pine wood chips were heated with water in a 10-1itre autoclave at a wood-to-liquor ratio of 1:4 under different conditions to produce three batches of differently prehydrolyzed chips. The prehydrolysis liquor was drained and the chips were used for the following examples without washing. The results of these prehydrolysis steps are set forth in the table below, indicating the prehydrolysis conditions to which the several batches of chips were subjected:

TABLE II Batch Maximum temperature, C 150 170 170 Time for raising the temperature from 20 C. to maximum temperature, min 102 126 126 Time at maximum temp, min 13 4 32 Chips prehydrolyzed as noted above were then used for various examples as to which data are set forth below, and, as noted, with varying charges and compositions of chemical liquor, varying digesting temperatures and times, etc. In the data below, the quantities of chemicals charged are based per kilogram of original bone dry wood.

EXAMPLE I Some of each of the three batches of prehydrolyzed chips were digested according to a conventional sulfate method of cooking or digestion, as noted above, at a wood-to-liquor ratio of 1:4. In each instance, the digestion temperature was raised from to 170 C. in about 215 minutes, and was maintained at this higher temperature for about minutes. The unbleached pulp resulting from each one was analyzed as follows:

EXAMPLE II Prehydrolyzed wood chips from batch C of Table II were digested in accordance with this invention with different charges of sodium hydroxide and sodium sulfide and, inevitably, some sodium carbonate. A wood-to-liquor ratio of 1:5 was used, with a heating cycle of 60 minutes time at a temperature of 130 C., after which the temperature was raised 175 C. in minutes where Pentosan, percent I R10 TABLE IV Run 2a 2b 2c NagS0 gmsJkgm Nag0O3, grns./kgm NazS, gms./kgm NaOH, grns./kgm Time at 175 Inin Liquor pH after cooki g Kappa number Viscosity, cps Screening reject, percent EXAMPLE III (3a) Utilizing an additional batch of untreated wood, pine chips were prehydrolyzed with water in a -litre autoclave through a heating cycle in which the temperature was raised to 170 C. in 95 minutes and maintained at that temperature for 100 minutes. After draining of the prehydrolysis liquor, the digester was charged with 250 gms. sodium sulfite, 30 gms. sodium carbonate, 80 grns. sodium sulfide and 70 grns. sodium hydroxide, all based per kilogram of original bone dry wood. A woodto-liquor ratio of 1:3.5 was used with a heating cycle of heating from 80 C. to 170 C. in 90 minutes, and maintaining the temperature at 170 C. for 180 minutes. After washing, the pulp was screened, the screening rejects were 0.5% and the pulp, in a yield of 35% of bone dry wood, analyzed as follows: Kappa No. 25, viscosity 35 cps., 0.8% pentosans, R10 and R18 equal to 97.9 and 98.6 respectively.

(3b) In another experiment all conditions were exactly alike with the exception that the amount of sodium sulfite was lowered to 150 gms. per kilogram of original bone dry wood. After the digestion it was not possible to defibrate the pulp. Increasing the digestion time at 170 C. to 300 minutes still gave a undefibratable pulp.

(3c) For purposes of comparison another batch was prehydrolyzed exactly under the same, rather severe conditions and digested according to the sulfate process, charging the digester with 40 gms. sodium carbonate, 80 grns. sodium sulfide and 140 grns. sodium hydroxide. After 100 minutes at 170 C. the pulp was screened, the screening rejects were 0.6% and the pulp, in a yield of 33%, analyzed as follows: Kappa No. 25 and viscosity 35 cps. were the same as with the sulfite-sulfide-hydroxide digestion, but the pulp had 1.9% pentosans, R10 and R18 equal to 96.0 and 97.2 respectively.

Pulps from runs 3a and 3c in Example IV was treated in accordance with the following procedure to produce the following results.

TABLE V Pul according to Examples 3a and 3c:

Sodium hypochlorite at 20 0., pH 10.2, 60 m1n.:

Available chlorine gmsJk gm 30 0holoi'ination at 20 0., 30 min grns./k gn1 13 Alkaline washing at 0., min.:

30 Bleached pulp 3a (sulfate) Viscosity, cps 20 Pentosan, percent 1. 9 R10 95.0 97.0 Brightness, SCAN 011:62 90. 5 7

10 EXAMPLE IV (4a) Utilizing an additional batch of untreated wood, chips of pine sap-wood were prehydrolyzed with water at a temperature of 167 C. for minutes. After draining of the prehydrolysis liquor, the digester was charged with 55 gms. sodium sulfide and 150 gms. sodium hydroxide, both based per kilogram of original bone dry wood. A wood-to-liquor ratio of 1:3.5 was used with a cycle of heating from 80 C. to 167 C. in 4 hours, and maintaining the temperature at 167 C. for minutes. After washing, the pulp was screened. The results are summarized in the following Table VI.

(4b) In another experiment the prehydrolysis was performed as in the experiment 4a above. The cooking step used in this experiment was that of the second step of the Pat. 1,822,126. Thus, this experiment was carried out as in the Pat. 1,822,126 the sulfite cooking step of which being replaced of a prehydrolysis step in accordance with this invention. The digesting step was carried out utilizing a liquor providing 280 gms. sodium sulfite, gms. sodium sulfide and 140 gins. sodium hydroxide, all based per kilogram of original bone dry Wood. A wood-toliquor ratio of 1:3.5 was used with a rapid raising of the temperature to 140 C., and maintaining the temperature at 140 C. for 11 hours, the time necessary for obtaining a pulp. The results are summarized in the following Table VI.

(4c) This experiment was carried out as the experiment 4b. However, the temperature used in the second step was C. rather than 140 C. In this way a period of time at maximum temperature was reduced to 75 minutes.

(4d) This experiment was carried out in accordance with this invention, the prehydrolysis being identical with those in the examples 4a, 4b and 40. After draining of the prehydrolysis liquor, the digester was charged with 320 gms. sodium sulfite, .80 gms. sodium sulfide and 65 gms. sodium hydroxide, all based per kilogram of original bone dry wood. The temperature was raised to 170 C. and maintained at 170 C. for 200 minutes. The results are summarized in the following Table VI.

TABLE VI.--RESULTS OBTAINED Kappa Yield, Viscosity, Pentosan, Exper.No. Number percent cps. percent R10 R18 The only experiment given appreciable amounts of screenings was experiment 4c having an amount of screenings above 10%.

From this table it is evident that a process in accordance with this invention (experiment 4d) gives a pulp having lower pentosan and higher values for R10 and R18 than the known process of prehydrolysis and sulfate cooking and a combination of a prehydrolysis step and a second digesting step in accordance with the Pat. 1,- 822,126. In order to reduce the long cooking time when using the prehydrolysis step and the second digesting step in accordance with the Pat. 1,822,126 the temperature was raised in the experiment 40. However, the results were about the same as when cooking at 140 C. However, the amount of screening were appreciable.

EXAMPLE V In this experiment two other species of wood were treated in accordance with this invention. These species were spruce and birch respectively. The prehydrolysis was carried out at 167 C. with water and the second step was carried out by heating the cooking liquor to 170 C. in 5.5 hours and maintaining at 170 C. for 3 hours. The wood-to-liquor ratio was 1:4. The periods 1 1 of time used and the liquors used as well as the results are summarized in the following Table VII.

The higher pentosan content in the birch pulp is due to the composition of the wood used. When cooking birch with exactly the same prehydrolysis followed by a sulfate cook there is obtained a pentsoan content of 3.9% and the viscosity of 50 cps. Thus, there is also in this case obtained a substantial improvement of the pulp.

As will be apparent from the foregoing examples and the discussion heretofore, the cooking cycle times and temperatures, as well as the charge of cooking chemicals, can be varied over fairly wide ranges in accordance with this invention. Thus, although it may be possible to charge about 300-600 gms. sulfite per kg. of bone dry wood, this chemical composition may satisfactorily be reduced down to as little as 250 gms. per kg. in accordance herewith, provided that there is sufiicient sulfide and hydroxide present. As a practical matter, and for primarily economic reasons rather than technological ones, the upper limit for sodium sulfite in accordance herewith should be considered as about 600 gms. per kgs. wood, although little if any technological advantage is achieved by using more than 400 gms. per kgs. wood. From the data regarding Example I, it will be understood that a variation of the prehydrolysis technique provides pulps having considerably lower pentosan contents than those obtained by conventional sulfate cooking, with or without a prehydrolysis step.

Reviewing the data reported in connection with the various examples indicates that, not only can a chemical charge for digesting liquor be varied within wide limits, but also unexpectedly high viscosities are obtained in accordance herewith to produce extremely low pentosan contents if the cooking liquor contains sodium sulfide and hydroxide in combination with sodium sulfite.

All the foregoing data and description relating to this invention indicate, as will be understood, the importance of a digestion in a liquor containing sodium sulfite, sodium sulfide and sodium hydroxide as set forth herein, especially to produce enhanced advantages for producing highpurity dissolving pulps, in accordance herewith, having high viscosities and low contents of pentosans and alkalisoluble cellulose. Satisfactory results are achieved in accordance herewith when the proportion of sodium sulfide is at least about 40 grns./kgs. bone dry wood and the proportion of sodium hydroxide is at least about 20 gms./ kgs. bone dry wood. From FIG. 1 it is clear that both sodium sulfide and sodium hydroxide must be present in order to achieve a satisfactory pulping rate. That both these chemicals must be present to obtain a high viscosity is clear from FIG. 4. From FIG. 4 it is also clear that the lower limit for the proportion of active alkali, i.e. the sum of sodium sulfide and sodium hydroxide, is about 60 gms./kgs. bone dry wood. The ratio of sulfide to hydroxide is not very critical. However, critical lower limits are 40 gms. sulfide and 20 grns. hydroxide per kg. bone dry wood in accordance with FIG. 5. There is no distinct upper limit for the amount of active alkali. From FIGS. 2 through 4 it is evident that optimal properties normally are obtained when using 75l25 gms. active alkali per kg. bone dry wood. Above this amount of active alkali the results are gradually impaired. However, for practical reasons including the cooking time a preferred amount is -150 gms./kg. wood. Little or nothing will be gained when using 'more than 180 gms. of active alkali per kg. wood. 'For practical reasons an upper limit of 16 0 or 170 gms./ kg. wood is often preferred.

From the data regarding Example II, it can be concluded that small proportions of free alkali as sodium hydroxide have little influence on alkali solubility and pentosan contents but help considerably to reduce the cooking time. Runs 2a and 2b, with 25 and 50 gms. per kgs. of sodium hydroxide, have given very satisfactory analytical data, particularly viscosity and kappa number. At the higher level of free alkali in run 20, however, the pentosan content goes up and the R10 and R18 go down, but, nevertheless, the analytical figures are much better than the corresponding sulfate cooking or digesting run 10.

As will be understood from the foregoing, cooking or digesting treatments of prehydrolyzed wood in accordance herewith, although, under widely varying conditions, produce dissolving pulps, having pentosan contents considerably lower than those obtained by digesting the same prehydrolyzed chips by a conventional sulfate cooking technique and/or by sulfide-reinforced sulfite paper pulp techniques. As noted, also, the prehydrolysis step in accordance herewith is satisfactorily carried out with other temperatures and with steam instead of water. In the charge ranges noted in the examples, a part of the chemicals remained unchanged after the alkaline cooking. Accordingly, substantial parts of the spent liquor from the digestion steps are available for re-use in preparing new cooking liquor, thereby reducing the consumption of chemicals.

For obtaining the greatest economic advantages of the invention, as will be understood, recovery of the chemicals is important, and is readily accomplished by conventional and well known means for recovering sodium and sulfur materials from spent cooking liquors in the form of sodium sulfite and sodium sulfide along the lines of conventional paper-making pulping techniques, even though this invention is primarily related to dissolving pulps and not paper making pulps.

While the methods and compositions herein described form preferred embodiments of this invention, this invention is not limited to these precise methods and compositions, and changes may be made therein without departing from the scope of this invention which is defined in the appended claims.

What is claimed is:

1. In a process for the production of high-purity cellulosic dissolving pulps from wood wherein fibrous starting materials are first hydrolyzed and then digested in a liquor containing sodium sulfite, sodium sulfide and sodium hydroxide, the combination of steps whereby pure product having high cellulose content, high viscosity, low pentosan content and low alkali resistance is produced, comprising: hydrolyzing said fibrous starting materials with water at -180 C. for a sufiicient period of time to dissolve 10- 25% of the wood thereby substantially dissolving the hemicellulose components of said starting material and leaving lignin components substantially unaffected and then digesting at a temperature of 180 C. the hydrolyzed fibrous materials in an alkaline liquor containing, per kilogram of bone dry fibrous starting material,

(A) 150 to 600 grams sodium sulfite,

(B) at least 40 grams sodium sulfide, and

(C) at least 20 grams sodium hydroxide, the sum of (B) and (C) being within the range of 60180 grams, thereby substantially dissolving said lignin components.

2. The process according to claim 1 wherein the prehydrolysis with water is carried out for sufficient period 13 of time to dissolve 12-15% of the wood in the case of soft wood.

3. The process according to claim -1 wherein the prehydrolysis with water is carried out for sufficient period of time to dissolve 15-25% of the wood in the case of hard wood.

4. The process of claim 1 wherein the digesting liquor contains 130-150 gms. sodium sulfide and sodium hydroxide per kg. bone dry fibrous starting material.

References Cited UNITED STATES PATENTS HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 

